GE B90 Instruction Manual

Also see for B90: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

CHAPTER 8: APPLICATION OF SETTINGS ENHANCING RELAY PERFORMANCEB90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 8-118Table 8-9: North bus differential protection settingsTable 8-10: South bus differential protection settings8.6 Enhancing relay performance8.6.1 Using setting groupsIn the example of the South bus, CT-2 is the weakest (most prone to saturation) CT dictating values of some settings.However, CT-2 may not be a part of the South bus protection zone if the S-2 switch is opened. As the position of the switchmust be provided for the dynamic bus replica, the status of the switch can be re-used to control the setting groups andapply more sensitive settings if the weakest CT is not part of the bus zone at a given time. For example, if the S-2 switch isopened while the S-6 switch is closed, the CT-4 becomes the weakest CT connected to the South bus. The higherbreakpoint (HIGH BPNT) could be increased to 22.88 pu (fourth column of the Limits of Linear Operations of the CTs table).Setting Value CommentsPICKUP 0.1 pu Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The pu value is for the base of 1200A. This means the actual pickup is 120 Aprimary.LOW SLOPE 25% Default value. Lower or higher values can be entered upon security/dependabilityrequirements.LOW BPNT 1.79 pu None of the CTs saturate for AC currents below 1.79 pu even with 80% remanence. The DCcomponent, however, combined with the remanence can saturate some CTs even for currentsbelow 1.79 pu. The B90 copes with saturation using the current directional principle.HIGH SLOPE 60% Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The value of 60% ensures that the differential characteristic alone (without thedirectional principle) work correctly under AC saturation of the CTs (26% of spuriousdifferential during the fault on C-1 saturating CT-1).HIGH BPNT 8.96 None of the CTs saturate for AC currents below 8.96 pu. The DC component, however, cansaturate some CTs even for currents below 8.96 pu. The B90 copes with saturation using thecurrent directional principle.HIGH SET 5.94 The maximum spurious differential current is 2.97 pu. Due to limited accuracy of analysis andthe effect of DC saturation a security factor of 2 has been adopted. The highest internal faultcurrent is 14kA, or 11.67 pu giving a good chance to clear a number of faults by the unbiaseddifferential operation.Setting Value CommentsPICKUP 0.1 pu Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The pu value is for the base of 1200A. This means the actual pickup is 120 Aprimary.LOW SLOPE 25% Default value. Lower or higher values can be entered upon security/dependabilityrequirements.LOW BPNT 1.83 pu None of the CTs saturate for AC currents below 1.83 pu even with 80% remanence. The DCcomponent, however, combined with the remanence can saturate some CTs even forcurrents below 1.83 pu. The B90 copes with saturation using the current directional principle.HIGH SLOPE 60% Default value. Lower or higher values can be entered upon security/dependabilityrequirements. The value of 60% ensures that the differential characteristic alone (without thedirectional principle) work correctly under AC saturation of the CTs (24% of spuriousdifferential during the fault on C-2 saturating CT-2).HIGH BPNT 9.13 pu None of the CTs saturate for AC currents below 9.13 pu. The DC component, however, cansaturate some CTs even for currents below 9.13 pu. The B90 copes with saturation using thecurrent directional principle.HIGH SET 5.54 The maximum spurious differential current is 2.77 pu. Due to limited accuracy of analysis andthe effect of DC saturation a security factor of 2 has been adopted. The highest internal faultcurrent is 14kA, or 11.67 pu giving a good chance to clear a number of faults by the unbiaseddifferential operation.